Method of raising the content of nitrogen and oxygen in titanium

ABSTRACT

A METHOD OF RAISING THE CONTENT OF NITROGEN AND OXYGEN IN TITANIUM BY ADDING NITRITES OF NITRATES OF NON-ALLOYING METALLIC ELEMENTS TO THE SPONGE BEFORE IT IS MELTED. THE PREFERRED COMPOUNDS TO BE ADDED ARE NANO2 AND NANO3. MODERN SPONGE IS LOW IN NITROGEN AND OXYGEN. ADDITIONS OF BOTH THESE ELEMENTS ARE NEEDED TO INCREASE THE STRENGTH OF THE PRODUCT.

United States Patent Office 3,625,679 Patented Dec. 7, 1971 3,625,679 METHOD OF RAISING THE CONTENT OF NITROGEN AND OXYGEN IN TITANIUM Howard B. Bomberger, Jr., Canfield, Ohio, assignor to RMI Company, Niles, Ohio No Drawing. Filed Apr. 23, 1970, Ser. No. 31,400 Int. Cl. C22b 53/00; C22c 15/00 US. Cl. 75175.5 4 Claims ABSTRACT OF THE DISCLOSURE A method of raising the content of nitrogen and oxygen in titanium by adding nitrites of nitrates of non-alloying metallic elements to the sponge before it is melted. The preferred compounds to be added are NaNO and NaNO Modern sponge is low in nitrogen and oxgyen. Additions of both these elements are needed to increase the strength of the product.

This invention relates to an improved method of raising the content of nitrogen and oxygen in titanium. The term titanium, when used herein without further qualification, includes either the unalloyed metal, or various titanium-base alloys, such as the alloy which contains 4% aluminum and 6% vanadium (Ti6Al-4V).

Nitrogen, oxygen, carbon and hydrogen dissolve interstitially in titanium and are referred to as interstitial elements. Among these elements, nitrogen and oxygen show similar effects on titanium. Although the interstitial elements are looked on as impurities in titanium, small amounts of nitrogen and oxygen contribute to the strength of a titanium product. If the content of either is too high, there is a loss of toughness, impact strength and ductility. Specifications on titanium alloy products usually permit a maximum of either 0.03% or 0.05% by weight nitrogen and 0.20% oxygen.

Titanium produced early in the period since it has become a commercial material had a high content of interstitial elements. These elements remained as impurities in the reduced sponge. Commonly the combined content of nitrogen and oxygen was as high as 0.2% by weight. The nitrogen in particular was above the optimum level. By contrast titanium sponge produced under modern practice is very low in both nitrogen and oxygen (commonly about 0.01% nitrogen and about 0.07% oxygen). Where a titanium product requires more oxygen to furnish the strength desired, the practice has been to add TiO to the sponge before it is melted. However, the maximum oxygen content allowed by specifications may be reached without the product attaining the full strength desired. The strength deficiency might be overcome by raising the nitrogen content closer to the allowable limit, but to my knowledge there has been no satisfactory way heretofore of adding nitrogen.

My earlier Pat. No. 2,433,626 describes a method of adding interstitial strengthening elements to titanium in which the element is added in the form of a compound of an alloying element, such as V This method is successful for adding oxygen, but less so for adding nitrogen. The difficulty is that nitrides of most alloying elements are highly refractory and tend not to dissolve, but re main as hard particles detrimental to the product.

An object of my invention is to provide an improved method of raising the content of both nitrogen and oxygen iii titanium and overcoming difiiculties encountered heretofore in adding nitrogen. A more specific object is to provide a method of rais ihg the content of nitrogen and oxygen in titanium in which I add to titanium sponge a nitrite or nitrate of a non-alloying metallic element, such as NaNO and NaNO According to my invention, I add to titanium sponge before it is melted a quantity of a nitrite and/or a nitrate of a non-alloying metallic element calculated to raise the nitrogen content of the ingot to the desired level. My preferred non-alloying element is sodium, but I can use other alkali metals or alkaline earth metals. Nitrites and nitrates also supply oxygen to the ingot, thereby lo wering the quantity of TiO needed or eliminating TiO' addition altogether. NaNO contains 20.3% nitrogen and 46.4% oxygen, while NaNO contains 16.5% nitrogen and 56.5% oxygen. I have found that about 60 to of the nitrogen and of the oxygen from either compound added to the sponge are recovered in the titanium ingot. The small amount of sodium or other non-alloying element vaporizes and essentially passes from the melting furnace. Metallic sodium commonly is used as a reducing agent in producing titanium sponge; hence sodium does not contaminate titanium. The quantity of NaNO- needed to raise the nitrogen content of a titanium ingot by each 0.01% is about 1 to 2 ounces per 100 pounds of sponge. The quantity of NaNO;; needed is about 1.5 to 3 ounces per 100 pounds. These quantities of NaNO and NaNO raise the oxygen content of the ingot by about 0.03% and 0.06% respectively. I have observed that, for each 0.01% increase in nitrogen content up to about 0.05%, both the yield strength and the ultimate tensile strength of a titanium product are increased by about 1000 to 2000 psi Following are specific examples which demonstrate how my invention operates:

EXAMPLE I Five 20-pound ingots of Ti6Al-4V alloy were melted to demonstrate the feasibility of adding nitrogen by means of NaNO and NaNO In this work, an attempt was made to raise the level of nitrogen from 0.007% in the starting material to the more desirable levels of 0.02 to 0.03%. Table I gives the results of this Work, including the weights of NaNO and NaNO added, the total level of nitrogen obtained in the final alloy, and the percent of nitrogen recovered from the additions made. This work demonstrated that nitrogen additions can be made by the addition of a nitrate or nitrite. The results also demonstrated that normal oxygen levels for the alloy can be obtained. In this case, a portion of the desired oxygen level of 0.17% was obtained by means of the nitrite or nitrate and the required balance by means of conventional TiO additions. The oxygen recovery was essentially 100% of the total oxygen added.

TABLE I.NITROGEN AND OXYGEN ADDITIONS AND CHEMICAL ANALYSES FOR 20-LB. Ti-6Al-4V INGOIS Sodium nitrite was employed as a means to increase the nitrogen level of a production heat of Ti-6Al-4V alloy. In this example, the nitrogen content was increased from a level of 0.011% to about 0.03% by the addition of 2.72 ounces of sodium nitrite to each of the individual briquette charges (weighing pounds). In addition to the oxygen contributed by the nitrite, oxygen was supplied by including 1.12 ounces of titanium dioxide in each briquette charge. The pressed blocks or briquettes were then welded into a consumable electrode for arc melting in the conventional manner. The ingot size, after the double melting, was 30 inches in diameter and it 3 weighed 9,720 pounds. The nitrogen and oxygen contents of the ingot were uniform and met quite satisfactorily the intended aims of 0.03% N and 0.17% O, as shown in Table II.

TABLE lL-CIIEMICAL ANALYSIS OF UJLZO'LB. Ti fiAl iV From the foregoing description it is seen that my invention affords a simple effective method of raising the content of both nitrogen and oxygen in titanium. Although raising the oxygen content has not been a problem, my invention enables the content of both nitrogen and oxygen to be raised as desired with the addition of a single material to the sponge.

I claim:

1. A method of raising the content of nitrogen and oxygen in titanium comprising adding to titanium sponge a compound of a non-alloying metallic element, said compound being selected from the group which consists of alkali metal and alkaline earth metal nitrites, nitrates 4 and combinations thereof, melting the sponge, and vaporizing said element.

2. A method as defined in claim 1 in which said element is sodium.

3. A method as defined in claim 1 in which said com' pound is NaNO and is added in an amount of about 1 to 2 ounces per 100 pounds of sponge for each 0.01 percent increase in nitrogen content of the melting product.

4. A method as defined in claim 1 in which said compound is NaNO and is added in an amount of about 1.5 to 3 ounces per 100 pounds of sponge for each 0.01 percent increase in nitrogen content of the melted product.

References Cited UNITED STATES PATENTS 2.554,031 5/1951 Jaffee et a1 75175.5 2,691,578 10/1954 Herres et a1 75175.5 2,703,278 3/1955 Finlay et al. 75l7 5.5 2,819,194 1/1958 Herres et al 75175.5 X 3,433,626 3/1969 Bomberger 75177 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

